In a power supply such as a battery, the voltage thereof changes depending on the amount of a residual energy, the ambient temperature, and the weight of a load of an electronic device to be driven. However, it is desired to supply a stable voltage to the electronic device. As a circuit that supplies the stable voltage to the electronic device from the power supply in which the voltage thereof changes, there is known a switching power supply circuit such as a DC-DC convertor.
As an example of the switching power supply circuit, an AC adapter is known. The AC adapter uses a commercial power supply as an input, and supplies the power supply voltage to the load through the transmission line (transmission cable).
Further, there is also known another switching power supply circuit built in a personal computer or the like, and charges a battery built in a digital camera though a transmission line such as a USB cable by connecting the transmission line to the personal computer and the digital camera.
Furthermore, as an in-vehicle use, there is also known further another switching power supply circuit built in a car accessory such as a car navigation, a car audio, or the like, and charges a battery built in a multifunction mobile phone such as a smart phone or an information terminal through a transmission line inside the vehicle such as a cable of several meters by connecting the transmission line to the multifunction mobile phone or the information terminal.
In a switching power supply circuit that supplies a power supply voltage to the load through the transmission line in this manner, a voltage drop occurs due to the resistance of the transmission line. As a result, a power supply voltage actually supplied to the load is made lower than a power supply voltage that should be originally supplied thereto. Therefore, it is demanded to output a voltage higher than a power supply voltage that should be supplied to the load by a voltage that will drop in the transmission line.
Especially, in a charging system that charges a battery or the like through the transmission line, a current in the order of several amperes flows through the transmission line, and a voltage drop in the transmission line is in the order of several hundred millivolts. Therefore, it is important for the switching power supply circuit to output the voltage higher than a power supply voltage that should be supplied to the load by the voltage that will drop in the transmission line.
Patent Document 1, for example, describes the switching power supply circuit in which a voltage that will drop in the transmission line is generated, the generated voltage is added to the power supply voltage that is desired for the load, and the added voltage is output to the transmission line in this manner.